This invention relates to "voice mail" systems for recording and playing back audio messages for use by a plurality of users.
A "voice mail" system is a record and playback system that stores messages intended for one of a plurality of users. Substantially all presently known voice mail systems which can be used by more than a few users are digital, disk-based systems. An example of such a system is shown in U.S. Pat. No. 4,371,752 to Matthews, et al. In the Matthews, et al. system, a caller is greeted and instructed by prompt signals, to which the caller can respond by entering Touch-Tone commands. The caller can then record a message, review messages, or perform a number of other operations. Recorded messages are digitized and stored in a disk memory and are then associated with a recipient's mailbox address. To deliver the message, the system places periodic calls to the recipient using a private branch exchange (PBX) to which the system is connected. If the recipient answers such a call, the recipient is able, by entering tone commands, to review the message. Alternatively, the recipient can phone the voice mail system from a PBX or external line and inquire whether any messages are waiting.
The Matthews, et al. system is both complex and expensive. Its technique of calling recipients periodically to inform them of waiting messages makes it poorly suited for smaller, simpler installations in which a PBX line cannot be spared for such inefficient use. This technique also increases the overhead of such a system's central processing unit, further increasing cost.
Many voice mail systems use Touch Tone tones transmitted by a user for system control purposes. Such tones are typically decoded by a tone decoder. Sometimes non-tone signals, such as prompting messages sent by the system, can accidentally trigger the decoder and initiate an undesired operation. One prior art method for eliminating such undesired responses is to use a delay circuit that requires the tones to be detected for a relatively lengthy period, such as one second, before triggering a response. This method, of course, slows the system response.
Another problem associated with tone commands is that if a tone command is entered while a message is being recorded, the tone command will be recorded with the message. The recorded tone will then cause the command to be re-executed when the message is played back from tape. Solutions to this problem which involve filtering the tones from the tape record head input are ineffective due to the number and the wide frequency range of the component tones.
An article in EDN Magazine describes an approach for increasing the dynamic range of a low-speed, full duplex modem which transmits output signals at a first transmit frequency or tone and receives input signals at a second receive frequency or tone. The approach illustrated by this article presupposes that a filter is used to substantially remove the transmit tone from the receiver input. A hybrid interface is then used to further improve performance of the modem at the receive tone frequency. The hybrid interface includes differential amplifiers to help separate the transmitted output signals from the received input signals. (Peter Single, Optimize the Hybrid Interface to Increase Modem Dynamic Range, EDN, Oct. 18, 1984, pp. 279-285.)
This modem approach is ill-suited to voice mail applications. The Touch-Tone signals used in such applications extend over a broad frequency bandwidth, and thus a filter cannot satisfactorily be used to remove the unwanted prompt signals from the input of the tone decoder. Similarly, because the potentially interfering prompt signals extend over a broad bandwidth, the circuit of this article which separates transmitted from received tones cannot be optimized for a single frequency. Instead, it would have to be made to perform well throughout the full audio spectrum over which the interfering prompt signals might range.